Some six and nine coordinated complexes of trivalent lanthanide metal ions with 4[N-(2'-hydroxy-1'-naphthalidene)amino]antipyrinesemicarbazone (HNAAPS) with the general composition LnX3.n(HNAAPS) [X = NO3, n = 1; X = NCS or ClO4, n = 2; Ln = La, Pr, Nd, Sm, Gd, Tb, Dy or Ho] have been isolated. All the complexes have been characterized on the basis of analytical data, molar conductance, magnetic susceptibility, electronic and infrared spectral measurements. The ligand HNAAPS behaves as neutral tridentate (N, N, O) ligand. The coordination number of the central metal ion is either six or nine in these complexes. Thermal properties of these complexes were also investigated. KEY WORDS: Lanthanide(III) complexes, 4[N-(2'-hydroxy-1'-naphthalidene)amino]antipyrine semicarbazone Bull. Chem. Soc. Ethiop. 2006, 20(1), 167-172.